Illumination systems are known per se and are used to provide light in both functional applications and decorative applications. In functional applications, such as street lamps, ceiling- and wall-mounted lighting fixtures, the light is typically provided to counteract reduced visibility and the light output is therefore configured accordingly. The requirements on the light output may include parameters such as wavelength spectrum, color point or color temperature, intensity, solid angle, light area, luminous power, luminance etc. Generally, the wavelength spectrum relates to the color of the light. In illumination applications such as general lighting or product highlighting, the color rendering properties of the light generally is an important requirement on the light output, which relates to the wavelength spectrum, and which generally is expressed as the general color rendering index (CRI). In decorative applications, such as ceiling and wall-mounted light tiles, poster boxes, signs and logos, the required light output is mainly determined on aesthetic grounds. Also known are illumination systems which configure the output light to satisfy both these functional and decorative requirements to varying degrees. Illumination systems may also be used in, for example, projection systems such as a digital projector, or so-called beamer, for projecting images or displaying a television program, a film, a video program or a DVD, or the like. In display applications, the color gamut of the system generally is an important requirement on the spectral composition of the light output.
Illumination systems where the light is provided for use by an associated object are also known. For example, liquid crystal display devices, also referred to as LCD display, where the illumination system is a backlight and the associated object is a non-emissive LCD panel. Such display devices are used, for example, in television receivers, (computer) monitors, (cordless) telephones and portable digital assistants. Typically, the output light requirements of a display device specify the light reflected and/or transmitted by the associated object—consequently the optical characteristics of the associated object need to be taken into consideration when the light-output requirements for the illumination system are determined.
In some of these applications, such as a backlight for use in an LCD display, the requirements on the light output of the illumination system may be quite strict, and available light-emitting devices may not meet these requirements sufficiently. This is also true for light-emitting diodes (LED's), which are considered attractive semiconductor light-emitting devices for use in a backlight because of their energy efficiency and compact size. To help achieve the light output requirements, luminescent elements are frequently employed to absorb the photons, produced by the light-emitting device, which have a certain wavelength spectrum, and to emit photons having a different wavelength spectrum. Phosphors are a well-known example of luminescent elements, and the process of absorption and subsequent emission is often referred to as conversion, or wavelength conversion.
An additional problem is that the optical characteristics of the associated object may vary—for example, LCD panels may be produced using different techniques and different materials by different manufacturers—which means that the output light requirements on the associated illumination system must also vary. In other words, one illumination system, for example, one backlight, may not be suitable for the whole range of associated objects even when luminescent elements are used.
For example, U.S. Pat. No. 7,052,152 discloses in FIG. 12A an LCD panel using a backlight, in which a phosphor is employed on the cover plate of the backlight as a luminescent element. This has the advantage that the cover plate with phosphor can be matched to the LCD panel, and the rest of the backlight may be standardized in terms of light output. However, a drawback of the known illumination system with the phosphor on the cover plate is that variation in phosphor layer thickness causes spatial intensity and color variations in the light emitted by the phosphor. Such variations are noticeable by the user of the LCD panel and considered undesirable.